STEP1: 100 dollar and 120 dollar [assume you know 20% mean 20 dollar more] and so either 100 / 120 or 120/100 and if you try both, then you know it is 0.833 or 1.2;

STEP 2: 1-0.833 LOOKS the best possible answer because 1-1.2 doesn't make sense at all [price rise 20% and no way will dollar down exactly the same %, right ? and this common sense is based on mathematical experience!!]